Halliday & Matthiessen (2014: 178):
Modulated clauses, on the other hand, while they also occur frequently as offers, commands and suggestions (I’ll be going, you should be going, we ought to be going), regularly implicate a third person; they are statements of obligation and inclination made by the speaker in respect of others, e.g. John’s supposed to know that, Mary will help; …
Such statements of obligation function as propositions, since to the person addressed they convey information rather than goods-&-services. But they do not thereby lose their rhetorical force: if Mary is listening, she can now hardly refuse; and we know what happens if we don’t obey the law!